An injury to a blood vessel activates the haemostatic system that involves complex interactions between cellular and molecular components. The process that eventually causes the bleeding to stop is known as haemostasis. An important part of haemostasis is coagulation of the blood and the formation of a clot at the site of the injury. The coagulation process is highly dependent on the function of several protein molecules. These are known as coagulation factors. Some of the coagulation factors are proteases which can exist in an inactive zymogen or an enzymatically active form. The zymogen form can be converted to its enzymatically active form by specific cleavage of the polypeptide chain catalyzed by another proteolytically active coagulation factor. Factor VII is a vitamin K-dependent plasma protein synthesized in the liver and secreted into the blood as a single-chain glycoprotein. The Factor VII zymogen is converted into an activated form (Factor VIIa) by specific proteolytic cleavage at a single site, i.e. between R152 and I153 of the Factor VII sequence (wild type human coagulation Factor VII) resulting in a two chain molecule linked by a single disulfide bond. The two polypeptide chains in Factor VIIa are referred to as light and heavy chain, corresponding to residues 1-152 and 153-406, respectively, of the Factor VII sequence. Factor VII circulates predominantly as zymogen, but a minor fraction is on the activated form (Factor VIIa).
The blood coagulation process can be divided into three phases: initiation, amplification and propagation. The initiation and propagation phases contribute to the formation of thrombin, a coagulation factor with many important functions in haemostasis. The coagulation cascade starts if the single-layered barrier of endothelial cells that line the inner surface of blood vessels becomes damaged. This exposes subendothelial cells and extravascular matrix proteins to which platelets in the blood will stick to. If this happens, Tissue Factor (TF) which is present on the surface of sub-endothelial cells becomes exposed to Factor VIIa circulating in the blood. TF is a membrane-bound protein and serves as the receptor for Factor VIIa. Factor VIIa is an enzyme, a serine protease, with intrinsically low activity. However, when Factor VIIa is bound to TF, its activity increases greatly. Factor VIIa interaction with TF also localizes Factor VIIa on the phospholipid surface of the TF bearing cell and positions it optimally for activation of Factor X to Xa. When this happens, Factor Xa can combine with Factor Va to form the so-called “prothombinase” complex on the surface of the TF bearing cell. The prothrombinase complex then generates thrombin by cleavage of prothrombin. The pathway activated by exposing TF to circulating Factor VIIa and leading to the initial generation of thrombin is known as the TF pathway. The TF:Factor VIIa complex also catalyzes the activation of Factor IX to Factor IXa. Then activated Factor IXa can diffuse to the surface of platelets which are sticking to the site of the injury and have been activated. This allows Factor IXa to combine with FVIIIa to form the “tenase” complex on the surface of the activated platelet. This complex plays a key role in the propagation phase due to its remarkable efficiency in activating Factor X to Xa. The efficient tenase catalyzed generation of Factor Xa activity in turn leads to efficient cleavage of prothrombin to thrombin catalyzed by the prothrombinase complex.
If there are any deficiencies in either Factor IX or Factor VIII, it compromises the important tenase activity, and reduces the production of the thrombin which is necessary for coagulation. Thrombin formed initially by the TF pathway serves as a pro-coagulant signal that encourages recruitment, activation and aggregation of platelets at the injury site. This results in the formation of a loose primary plug of platelets. However, this primary plug of platelets is unstable and needs reinforcement to sustain haemostasis. Stabilization of the plug involves anchoring and entangling the platelets in a web of fibrin fibres.
The formation of a strong and stable clot is dependent on the generation of a robust burst of local thrombin activity. Thus, deficiencies in the processes leading to thrombin generation following a vessel injury can lead to bleeding disorders e.g. haemophilia A and B. People with haemophilia A and B lack functional Factor VIIIa or Factor IXa, respectively. Thrombin generation in the propagation phase is critically dependent of tenase activity, i.e. requires both Factor VIIIa and FIXa. Therefore, in people with haemophilia A or B proper consolidation of the primary platelet plug fails and bleeding continues.
Replacement therapy is the traditional treatment for hemophilia A and B, and involves intravenous administration of Factor VIII or Factor IX. In many cases, however, patients develop antibodies (also known as inhibitors) against the infused proteins, which reduce or negate the efficacy of the treatment. Recombinant Factor VIIa (Novoseven®) has been approved for the treatment of hemophilia A or B patients that have inhibitors, and also is used to stop bleeding episodes or prevent bleeding associated with trauma and/or surgery. Recombinant Factor VIIa also has been approved for the treatment of patients with congenital Factor VII deficiency. It has been proposed that recombinant FVIIa operates through a TF-independent mechanism. According to this model, recombinant FVIIa is directed to the surface of the activated blood platelets by virtue of its Gla-domain where it then proteolytically activates Factor X to Xa thus by-passing the need for a functional tenase complex. The low enzymatic activity of FVIIa in the absence of TF as well as the low affinity of the Gla-domain for membranes could explain the need for supra-physiological levels of circulating FVIIa needed to achieve haemostasis.
Recombinant Factor VIIa has a pharmacological half-life of 2-3 hours which may necessitate frequent administration to resolve bleedings in patients. Further, patients often only receive Factor VIIa therapy after a bleed has commenced, rather than as a precautionary measure, which often impinges upon their general quality of life. A recombinant Factor VIIa variant with a longer circulation half-life would decrease the number of necessary administrations and support less frequent dosing thus hold the promise of significantly improving Factor VIIa therapy to the benefit of patients and care-holders.
In general, there are many unmet medical needs in people with coagulopathies. The use of recombinant Factor VIIa to promote clot formation underlines its growing importance as a therapeutic agent. However, recombinant Factor VIIa therapy still leaves significant unmet medical needs, and there is a need for recombinant Factor VIIa polypeptides having improved pharmaceutical properties, for example increased in vivo functional half-life, improved activity, and less undesirable side effects.
Various methods have been employed in the development of a Factor VII polypeptide with prolonged circulatory half-life. Some of these methods relate to conjugation of Factor VII with water soluble polymers such as PEG (poly ethylene glycol). WO03031464 disclose an enzymatic approach where PEG groups can be attached to glycans present on the polypeptide.